1. Field of the Invention
The present invention relates to a crystal oscillator which comprises a crystal blank and an IC (integrated circuit) chip that has integrated therein an oscillation circuit using the crystal blank, both of which are contained in a surface mount package, and more particularly, to a surface mount crystal oscillator which ensures an area for accommodating an IC chip in a package.
2. Description of the Related Arts
Surface mount crystal oscillators, which have a quartz crystal blank and an IC chip that has integrated therein an oscillation circuit using the crystal blank, both of which are contained in a surface mount package, are built in a variety of portable electronic devices in particular as reference sources for frequency and time because of their small sizes and light weights. Particularly, a temperature compensated crystal oscillators are widely employed in mobile telephones and the like because such temperature compensated crystal oscillators provide a high frequency stability against variations in ambient temperature. A representative surface mount crystal oscillator is referred to as a one-chamber type which contains an IC chip and a crystal blank in the same recess formed in a package, for example, as shown in U.S. Pat. No. 6,720,837.
FIG. 1A illustrates a conventional surface mount crystal oscillator in a one-chamber structure. The illustrated crystal oscillator comprises IC chip 2 and crystal blank 3 accommodated in a recess formed in package body 1 of a surface mount type, and flat metal cover 4 placed over the recess to hermetically seal IC chip 2 and crystal oscillator 3. Package body 1 is formed of laminated ceramics which include flat and substantially rectangular bottom wall layer 1b, and frame wall layer 1a having a substantially rectangular opening. The opening formed through frame wall layer 1a defines a recess for containing IC chip 2 and crystal blank 3. A step is formed on the inner wall of frame wall layer 1a on one side, and a pair of crystal holding terminals 6 are formed on the top surface of the step for use in holding crystal blank 2.
Mounting terminals 5 are disposed at four corners of the outer bottom surface of package body 1 for use in surface-mounting the crystal oscillator on a wiring board. These mounting terminals 5 include a power supply terminal, a ground terminal, an output terminal for providing an oscillation output, and the like. Shield electrode 7 is also embedded in bottom wall layer 1b for electromagnetically shielding IC chip 2 from the wiring board. Shield electrode 7 is electrically connected to the ground terminal among mounting terminals 5.
IC chip 2 has electronic circuits, which include an oscillation circuit using crystal blank 3, integrated on a semiconductor substrate. The oscillation circuit is formed on one main surface of the semiconductor substrate by a general semiconductor device fabricating process. Here, a circuit forming surface will refer to one of the two main surfaces of IC chip 2 on which the oscillation circuit is formed. A plurality of IC terminals are also formed on the circuit forming surface for connecting IC chip 2 to external circuits. The IC terminals include a power supply terminal, a ground terminal, an oscillation output terminal, a pair of connection terminals for connection with crystal blank 3, and the like.
Circuit terminals are provided on the bottom surface of the recess in package body 1 in correspondence to the IC terminals. The circuit terminals corresponding to the power supply terminal, ground terminal, and oscillation output terminal on IC chip 2 are electrically connected to mounting terminals 5 through conductive paths routed on a plane in which frame wall layer 1a is laminated on bottom wall layer 1b, and on an outer surface of package body 1. Circuit terminals corresponding to the pair of connection terminals on IC chip 2 are electrically connected to the pair of crystal holding terminals 6 through via holes 11 extended through frame wall layer 1a at positions of the step. IC chip 2 is secured on the bottom surface of the recess by electrically and mechanically connecting the IC terminals to the circuit terminals through ultrasonic thermo-compression bonding using bumps 8, such that the circuit forming surface opposes the bottom surface of the recess in package body 1. In a temperature compensated crystal oscillator, IC chip 2 is provided with write terminals, which belong to the IC terminals, for writing temperature compensation data into a temperature compensation mechanism within IC chip 2. The write terminals are electrically connected to external write terminals, not shown, disposed on the outer side surface of package body 1.
Metal cover 4 is bonded to a metal ring provided on an open end face, that is, the upper surface of package body 1 which surrounds the recess in package body 1, through seam or beam welding, and is electrically connected to the ground terminal within mounting terminal 5 through via hole 11 extended through frame wall layer 1a. 
As illustrated in FIG. 1B, crystal blank 3, which comprises, for example, a substantially rectangular AT-cut quartz crystal blank, is formed with excitation electrodes 9a on both main surfaces, respectively. From these excitation electrodes 9a, lead-out electrodes 9b are extended toward both ends of one side of crystal blank 3, respectively. Crystal blank 3 is secured to crystal holding terminals 6 with conductive adhesive 10 at both ends of the one side thereof to which lead-out electrodes 9b are extended, thereby electrically and mechanically connecting crystal blank 3 to crystal holding terminals 6 and holding crystal blank 3 within the recess.
However, in the surface mount crystal oscillator in the foregoing configuration, the step is required on the inner wall of package body 1 for holding crystal blank 3 because crystal blank 3 is positioned above IC chip 2 in the recess of package body 1, and the recess is closed by flat metal cover 4. This results in a lower proportion of an area available for accommodating IC chip 2 to the outer planar dimensions of package body 1. Stated another way, assuming that IC chip 2 of the same size is contained, package body 1 is larger than that which is not provided with the step, thus constituting impediments to a reduction in size of the crystal oscillator. Particularly, the temperature compensated crystal oscillator requires larger IC chip 2 due to a temperature compensation mechanism integrated in IC chip 2, so that the resulting crystal oscillator has limitations to a reduction in size.
Japanese Patent Laid-open Application No. 2000-134037 (JP-A-2000-134037) discloses a crystal oscillator which includes a package body having a recess, an insulating cover formed in a concave shape, an IC chip secured in the recess of the package body, and a crystal blank secured in a concave portion of the insulating cover, wherein the insulating cover is secured to the package body with an anisotropic conductive adhesive to seal the crystal blank and IC chip without forming a step on the inner wall of the package. In this configuration, the elimination of the need for the step formed on the inner wall of the package body results in a higher proportion of an area available for accommodating the IC chip to the outer planar dimensions of the package body, thus making it possible to reduce the size of the crystal oscillator.
However, the crystal oscillator described in JP-A-2000-134037 experiences difficulties in maintaining the airtightness or sealablility in a space in which the crystal blank is sealed, and is prone to suffer from insufficient electric connections between the crystal blank and IC chip, because the insulating cover is secured to the package body with the anisotropic conductive adhesive.